PROCESS CONTROL

42 LTJ January 2009 No. 1 © 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

WOLFGANG HORNWolfgang Horn stud-ied physics at Technical University Darmstadt, Germany. In 1995 he joined the German Welding Institute (SLV) Mannheim, Germany, where he managed the process development department from 1997– 2002. In 2002 he joined DILAS Di-odenlaser GmbH where he is responsible for industrial laser systems.

●●Wolfgang Horn

DILAS Diodenlaser GmbHGalileo-Galilei-Straße 1055129 Mainz, Germany

Tel.: +49 (0)6131 9226-400E-mail: sales@dilas-ils.com

Website: www.dilas-ils.com

A Progressive Laser Joining MethodOnline Process Control With Pyrometer and Galvo Scanner

THE AUTHOR A newly designed galvo scanner combines the advantages of closed-loop temperature control by integrated on-axis pyrometer and fast beam deflection. It is used in quasi-simultaneous polymer welding as well as in selective reflow sol-dering where several solder joints have to be processed within the working area without using mechanical positioning. The temperature controlled processes are characterized by robustness and in-creased stability.

Pyrometer Controlled Laser Processes

Compared to other laser types, diode lasers convert the supplied energy (current) di-rectly into laser radiation. They allow quick modulation of the laser power which is es-sential for fast processes like closed-loop temperature control with pyrometers. Used for contour welding of polymers, soldering and heat treatment, the pyrometer sensor can be integrated into the processing optics and detects on-axis thermal radiation from the processed area.

To avoid interference between pyro -meter and laser source, the detector of the pyrometer must be sensitive for wavelengths different from the laser. Pyrometers utilized in materials processing mostly use detectors which are sensitive from 1800 nm–2100 nm whereas diode lasers are used at 808 nm or 980 nm wavelength.

To determine the absolute temperature by the process radiation, material properties like emission coefficient and surface charac-teristics need to be known. For most laser processes they are not determined. Du ring soldering, for example, the state of the sol-der and therefore its optical properties is changing from solid to liquid and back to solid again. In polymer welding the thermal radiation gets absorbed or scattered by filler material like glass, pigments or others.

FIGURE 1: Standard flat-field optic (left) shows different foci for pyrometer and laser whereas color-corrected flat-field lens (right) shows same focus [2].

For most applications a relative tempera-ture measuring is sufficient for either open or closed-loop process control. The controller of the pyrometer allows the storage of pro-cess data like welding temperature and laser output for documentation and analysis. It is therefore a useful tool for quality control and product development.

Galvo Scanners and Pyrometer

Using a galvo scanner is a common method when fast positioning or moving of the laser spot is needed. Typical applications are quasi-simultaneous welding of polymers or reflow soldering. The laser beam is moved by the mir-rors off the lenses’ optical axis and is no lon-ger parallel to it. This has some serious conse-quences for the usage of pyrometers. Optical properties like focal length and anti-reflective coatings of standard flat-field lenses are only working in a small dedicated wavelength range. The foci for the different wavelengths of pyrometer and laser are not congruent due to color aberration (see Figure 1). This means that during processing the pyrometer would detect radiation not from the laser focus but from somewhere else. Closed-loop proces-sing or even temperature monitoring would not be possible. With a special designed optic

it is now possible to have a color-corrected flat-field lens where pyrometer and laser fo-cus are congruent again. Figure 2 shows the DILAS galvo scanner DL.S20P with integrated single color pyrometer and color-corrected flat field lens. The scanner is designed to be used with COMPACT fiber-coupled diode la-

PROCESS CONTROL

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open loop and the burst pressure exceeds 11.7 bar.

The welding process is limited by the op-tical properties of the laser transparent part in the measuring range of the pyrometer. The transparency in the NIR determines the maximum achievable welding speed. Part dimensions are limited by the working field of the color-corrected flat field lens and the scanning unit.

Soldering

Besides of its advantages in electronic man-ufacturing, laser soldering with high-power diode lasers has all properties for contacting thin film solar cells. The solder joints which can be achieved by using cw diode lasers have several square millimeters (Figure 4). Laser soldering is a non-contact technology with an accurate and locally limited thermal input. This limits the thermal stress for the cell.

Usually Si solar cells are interconnected to strings which then get laminated into the modules. This technology requires a handling with additional equipment for the long and fragile strings. Using lasers, the string handling can be completely avoided by soldering directly through the laminate layers. The typical sequence for these mo-dules is glass, polymerized Ethylene Vinyl Acetate (EVA), tinned ribbons, solar cell,

ser systems and comes along with a sophis-ticated software package for galvo and pyrometer control.

Quasi-Simultaneous Welding

To examine the welding properties, a box-shaped test part has been designed which typically could be used in automotive indus-tries (Figure 3). The cover has been made of PBT (Polybutylene terephthalate) with 30 % glass content. The welding quality has been examined by bursting the boxes with com-pressed air.

In open loop the process window is quite narrow. With constant welding speed the la-ser power had to be stable within ± 2 % to achieve a maximum burst pressure of about 11.2 bar.

In closed loop the process temperature can be varied from 210°C to 280°C with-out significant influence on the welding result. The process window is wider than in

tinned ribbons and transparent PVF (Poly-vinyl fluoride) back sheet. Front and rear side of the PV module are transparent for laser radiation. Soldering can be done either before or after lamination. Concerning peel force and contact resistance, the quality of laser soldered joints exceeds other contac-ting technologies. The peel force is higher by a factor of 3, whereas the thermal resistance is only about 14 % [1].

The galvo scanner allows the soldering of all joints on the solar cell module with-out moving the optics or the cell. Due to the limited working field of the flat field lens, the galvo scanner has to be moved to process all cells of a module.

Figure 5 shows temperature and laser power versus time for a closed-loop solder-ing process. The temperature is raised for 150 ms and then kept constant for 200 ms to minimize the heat impact.

Conclusion

Together with the fast reacting laser sources, the newly developed optical components allow the combination of a closed-loop py-rometer control and fast beam positioning with galvo scanner. Tests show a more stable welding process with a wider process win-dow. The method offers new possibilities for industrial production to reduce scrap. The consisting documentation of relevant pro-cess parameters like temperature and laser power is now possible.

References

[1] W. Horn, High Power Diode Lasers for In-dustrial Applications, ICALEO (2007).

[2] E. Jaeger, Diode Lasers in Electronics and Plastics Production, 6th Workshop Appli-cation of High Power Diode Lasers, Dres-den, Germany (2006).

FIGURE 2: DILAS galvo scanner DL.S20P with integrated on-axis single color pyrometer for closed-loop temperature control

DILAS Diode Laser, Inc.

DILAS, the diode laser company, is fo-cused on delivering the most innovative technologies and advanced product solutions for the industrial, defense, graphic arts, and medical markets. Founded in 1994 in Mainz, Germany, with operations in Tucson, AZ, and Shanghai, China, DILAS designs, devel-ops and manufactures high-power semi-conductor laser components, modules and systems, including fiber-coupled products for worldwide distribution.www.dilas.com

THE COMPANY

FIGURE 3: Laser welded test box

FIGURE 4: Laser soldered contacts on solar cell module

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FIGURE 5: Temperature profile for closed-loop soldering